Examples of this kind of hollow member include an ICM housing for an incore monitor which serves to monitor the neutron flux generating in the RPV, and a control rod drive (CRD) housing for a control-rod drive which serves to drive the control rod.
The hollow member, e.g. the ICM housing, extends through a wall of the RPV into the coolant while it is welded to the wall of the RPV through a padding weld formed on an inner surface of the RPV wall. Such ICM housing is made of SUS304 stainless steel. In general, there exists in the weld a welding residual stress resulting from the welding heat input. Under the presence of the welding residual stress, corrosion proceeds markedly at the weld of the ICM housing in the coolant, resulting in cracking. This phenomenon is referred to as the stress corrosion cracking. Upon the occurrence of the stress corrosion cracking in the weld of the ICM housing, there is the possibility that the coolant leaks out of the RPV.
Once the stress corrosion cracking has occurred, repair may be conducted by a known method in which a shielding member is welded to the hollow member to surround the stress corrosion cracked portion as disclosed in JP-U-56-82696.
However, this kind of repairing method will add another new weld which is in danger of stress corrosion cracking. Further, this repairing method cannot be applied until the position where the stress corrosion cracking occurs is identified, that is, until the leakage of the coolant is detected. Therefore, it is impossible to prevent the leakage of the coolant or the occurrence of the stress corrosion cracking.